Solubility of radium sulfate and carbonate

in sodium chloride media

Artem V. Matyskin,1 Paul L. Brown,2 Christian Ekberg1 matyskin@chalmers.se 1 Nuclear Chemistry Industrial Materials Recycling group Chemistry and Chemical Engineering Department Chalmers University of Technology 2 Mineral Waste Management Rio Tinto Technology and Innovation

Hindåsgården 12.10.2016

Background

2

Radium in industrial wastes [ANN15]

3

Industry 226Ra specific activity, Bq∙kg-1 Uranium mining tailings 10000 - 140000

Phosphate fertilizers 75

Rare-earth metals industry wastes 33000

Fly ash produced by coal-fired power plant 142 - 605

Peat fly ash 200

Oil equipment 1∙106

Geothermal wastes 9000

[ANN15] Annual report 2015 SSM Swedish Radiation Protection Authority (SSM) and references therein

Radium thermodynamic properties

4

Radium has no stable isotopes, the only one long-lived is 226Ra with 1600 years half-life. Effective ionic radii of radium and barium in 8-coordination fold are 1.48 Å and 1.42 Å [SHA76]. Almost all thermodynamic properties of radium have been estimated using various theoretical approaches [LAN85].

[LAN85] Langmuir, D., & Riese, A. C. (1985). The thermodynamic properties of radium. Geochimica et cosmochimica acta, 49(7), 1593-1601. [SHA76] Shannon, R. T. (1976). Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallographica Section A: Crystal Physics, Diffraction, Theoretical and General Crystallography,32(5), 751-767.

RaSO4 solubility: experimental literature values

5

[Ra]2+, M [SO4]2-, M

RaSO4 pK’SP Media Ionic

strenght, M Temp

erature, K

Method Reference

1,07·10-6 5,1·10-5 10,31 5·10-5 M Na2SO4

2,1·10-4 293 Filtration Nikitin and Tolmatscheff 1933

1,45·10-7 5·10-4 10,28 5·10-4 M Na2SO4

1,56·10-3 293 Filtration and centrifugation Nikitin and Tolmatscheff 1933

3,06·10-8 5·10-3 10,2 5·10-3 M Na2SO4

1,5·10-2 293 Filtration and centrifugation Nikitin and Tolmatscheff 1933

1,12·10-8 5·10-2 9,25 5·10-2 M

Na2SO4

0,15 293 Filtration and centrifugation Nikitin and Tolmatscheff 1933

6,52·10-6 6,52·10-6 10,39 H2O 3,2·10-5 293 Dried and

crystallized RaSO4

Nikitin and Tolmatscheff 1933

- 2·10-2 10,52 2·10-2 M Na2SO4

6·10-2 283 Over- and undersaturation Hedström 2013

- 2·10-2 10,38 2·10-2 M Na2SO4

6·10-2 293 Over- and undersaturation Hedström 2013

2,22·10-8 2·10-2 9,95 2·10-2 M Na2SO4

6·10-2 303 Over- and undersaturation Hedström 2013

- 2·10-2 9,89 2·10-2 M Na2SO4

6·10-2 323 Over- and undersaturation Hedström 2013

- 2·10-2 9,71 2·10-2 M Na2SO4

6·10-2 343 Over- and undersaturation Hedström 2013

Experimental

6

Disassembly of radium source

7

Conversion of RaSO4(s) into RaCO3(s) for preparation of radium stock solution

8

Curie method:

RaSO4(s) → Ra2+(aq) + SO42-(aq)

Ra2+(aq) + CO32-(aq) → RaCO3(s)

Stock 2015: Total volume of 226Ra stock solution - 7 ml in 2.77M HCl Concentration of 226Ra – 0.4 mM/L

Stock 2016: Total volume of 226Ra stock solution - 14 ml in 0.1M HCl

Concentration of 226Ra – 3 mM/L

RaSO4 and RaCO3 crystal structure: XRPD, EXAFs and DFT study

9

Radium sulfate: • XRPD • DFT modelling

Radium carbonate: • XRPD • EXAFs

RaSO4 and RaCO3 Solubility in NaCl: experimental

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Radium sulfate: • Solubility from oversaturation at 0.3; 2; 3 M and at 4; 5 and 5.5M ionic

strength at 25.1 C • Na2SO4 was used as a source for SO4

2- ions and concentration of SO4

2- was 10 mM in all samples, NaOH was added • Sample volume was 2 mL, 0.16 mL of radium stock solution was

used (3.6 log units oversaturation compare to zero ionic strength)

Radium carbonate: • Solubility from oversaturation at 1.3; 1.9; 2.5 M and at 4 and 5M ionic

strength at 25 C • Na2CO3 was used as a source for CO3

2- ions and concentration of CO3

2- was 100 mM in all samples, NaOH was added • Sample volume was 2 ml with 0.645 ml of radium stock solution (3.3

log units oversaturation compare to zero ionic strength)

Results

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RaSO4 crystal structure

12

Reference a (Å) b (Å) c (Å) V (Å3)

This work 9.07 5.52 7.28 364.48

(Hedström et al., 2013) 9.07 5.52 7.28 364.48

(Weigel & Trinkl, 1968) 9.16 5.55 7.30 371.12

Atom Occupancy x y z Uiso

Ra(1) 1 0.18861 0.25000 0.15961 0

S(1) 1 0.45949 0.75000 0.22549 0

O(1) 1 0.58145 0.75000 0.13888 0

O(2) 1 0.36471 0.75000 0.10823 0

O(3) 1 0.37663 0.97373 0.30766 0

RaCO3 crystal structure

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N R, Å

Ra – O 9 2.885(5)

Ra···C 6 3.283(3)

Ra···O(I) 6 4.26(1)

Ra···O(II) 12 4.94(4)

Reference a (Å) b (Å) c (Å) V (Å3)

This work 5.46 9.18 6.69 335.32

(Weigel & Trinkl, 1972) 5.49 9.19 6.73 339.55

-1,6

-1,2

-0,8

-0,4

0,0

0,4

0,8

1,2

1,6

2 3 4 5 6 7 8 9 10

χ(k)

*k3

k/Å-1

Adopted extended Specific ion Interaction Theory (Hybrid SIT): free Ra2+ concentration

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][][][ 42 aqfree

apparenttotal RaSORaRa += +

aqRaSOSORa 424

2 →+ −+

−+ +→ 24

24 SORaRaSOsolid

−+ ⋅⋅⋅

⋅= −+

24

2

aq4

][][

][24

2RaSO40

SOfree

Rafree

aq

IP SORa

RaSOK

γγ

γ

−+ ⋅⋅⋅= −+24

2 ][][ 24

20SO

freeRa

freeSP SORaK γγ

−+ ⋅⋅⋅⋅= −+24

2 ][][ ][ 24

204 SO

freeRa

freeIP

aq SORaKRaSO γγ

004 ][ SPIPaq KKRaSO ⋅=

0002

2

][][ SPIPRa

freetotal KKRaRa ⋅+=

+

+

γ

( ) IIRaIaB

IAzKKRa RaRafreeiSPIP

total ⋅+−=⋅⋅+⋅⋅

−⋅− + ))log(()]log([1

][log 2102

200 εε

Adopted extended Specific ion Interaction Theory (Hybrid SIT): free [SO4]2- concentration

15

][][][ 4244

−− += NaSOSOSO freetotal

−−+ →+ 424 NaSOSONa

−+

−

−+

−

⋅⋅=

⋅⋅⋅= −+

−

24

-4

424

-4

4

NaSO)(2

4

NaSO40)( ][][

· ][

SONa

apparentNaSOIP

SOfree

NafreeNaSOIP K

SONa

NaSOK

γγ

γ

γγ

γ

freefreeapparentNaSOIP SONaKNaSO ][][ ][ 2

4)(44

−+− ⋅⋅= −

)][1(][][ )(244

4

freeapparentNaSOIP

freetotal NaKSOSO +− ⋅+⋅= −

( ) ( ) IIKIaB

IAzK NaNaNaSOIP

iapparentNaSOIP ⋅+−=

⋅⋅+⋅⋅

− −− ))log((log1

log 210

)(

2

)( 44εε

Extrapolation of logKsp of RaSO4 to zero I strength

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( ) ( ) IIKIaB

IAzK RaSPRaSPSP

iapparentSP ⋅+−=

⋅⋅+⋅⋅

− ))log((log1

log 210

2

εε

Extrapolation of logKsp of RaCO3 to zero I

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( ) ( ) IIKIaB

IAzK RaSPRaSPSP

iapparentSP ⋅+−=

⋅⋅+⋅⋅

− ))log((log1

log 210

2

εε

Comparison of apparent logKsp of RaSO4, BaSO4 and RaCO3, BaCO3

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Conclusion

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• Old radium sources were disassembled and RaSO4 was converted into RaCO3 using the Curie method to obtain radium solution.

• Solid RaSO4 and RaCO3 were analyzed using XRPD, DFT and EXAFS and crystal structure parameters were derived

• Solubilities of RaSO4 and RaCO3 were determined from oversaturation using NaCl as a background electrolyte to maintain constant ionic strengths

• RaSO4 solubility was extrapolated to zero ionic strengths and the value of -logKsp of 10.16±0.05 was obtained which is in agreement with literature values

• RaCO3 solubility was extrapolated to zero ionic strengths and the value of logKsp of 7.73±0.56 was obtained

• The shapes of radium sulfate and carbonate solubility curves are almost identical to the shapes of the corresponding barium solubility curves, which may indicate that radium undergoes similar specific ion interactions as barium in NaCl media

Future work: Complex formation constants of RaSO4aq and RaCO3

aq and undersaturation study of RaSO4 and RaCO3 solubilities

matyskin@chalmers.se